Longer Days Make Rats Experience SAD: New Insights into Depression-like Symptoms

During the winter months, people can feel blue due to less light during the day. In fact, the depression-like symptoms associated with seasonal affective disorder (SAD) have often plagued those who live in northern countries that experience little sunlight during the year. Now, researchers have discovered that for rats, it's just the opposite. Less sunlight makes them happier.

In order to make their findings, neuroscientists exposed rats for one week to 19 hours of darkness and five hours of light each day. They then exposed another set of rats for a week with the reverse--19 hours of light followed by a mere five hours of darkness. They discovered that, surprisingly, the rats that experienced more darkness had more nerve cells making dopamine--a hormone associated with less stress and anxiety. In contrast, the rats that experienced more light produced the neurotransmitter somatostatin, which made them more stressed.

Yet are the findings really all that surprising? Rats naturally explore and search for food at night, so it would make sense that they would be happier in the dark. Humans, in contrast, are wired for the daytime, which explains why we experience SAD during the winter months.

"We're diurnal and rats are nocturnal," said Nicholas Spitzer, a professor of biology at UC San Diego, in a news release. "So for a rat, it's the longer days that produce stress, while for us it's the longer nights that create stress."

They didn't just realize that the rats became more stressed at night, though. The researchers also discovered that the rat's brain cells adopt a new chemical code when subjected to large changes in the day and night cycle, flipping a switch to allow an entirely different neurotransmitter to stimulate the same part of the brain. More specifically, they found that this switch was not due to the growth of new neurons, but due to the ability of the same neurons there to produce different neurotransmitters.

Because rat brains are similar to human brains, the findings could provide great insight into the behavioral changes in our brain linked to light reception. It could also open the door for new ways to treat brain disorders such as Parkinson's, which is caused by the death of dopamine-generating cells in the brain.